First let me start off by saying that Acid attacks in that way that we see them on the news are new, very new, like 1967 new where the first acid throwing attacks were recorded in Bangladesh. (page 9)

Liquid Acid is widely documented as a form of torture during wartime, especially in recent conflicts in South/Southeast Asia such as during the Sri Lankan Civil War and the Khmer Rouge regime's reign in Cambodia. While its use as a method of torture is gruesome, its use as an actual weapon of war against opposing forces is not used due to severe limitations to its effectiveness.

First is that liquid acid is a very close range weapon and no nation has yet to develop artillery canisters or other delivery methods so far like what was used for mustard/chlorine gas in WWI, it's a similar statement to say why shotguns aren't widely used in frontline warfare. Ranged weapons are widely preferable over close quarters combat, and a bullet sure kills a lot quicker than acid.

Next is that after looking over some sources, the cost to produce nitric acid is expensive, and with it placed in the role as a very close quarters weapon or agent of torture in the context of war, the cost to mass produce it as a weapon most likely wouldn't be worth the investment.

To put it short, acid throwing in the way that we know it today is relatively new and not very viable as a weapon of war, but brutally effective as a method of torture, where it finds the use you're most likely looking for.

Though one form of acid widely used in warfare in a different context exists as Picric acid, which is very explosive and widely used in howitzer shells before and during WWI, through it was phased out over time with the rise of TNT as an explosive agent.

Although I don't have any sources with me to look over, it would also be very realistic to think that synthesized chemicals like nitric acid would indeed fall under the Geneva Convention banning chemical warfare, if anyone can confirm this it would be very appreciated.